Ability to download album and playlist

Add in the "meatball menu" of the Album and Playlist pages the option to
download the tracks of the corresponding album or playlist in a zip file.
When download is clicked, a dialog window appears allowing you to choose
the file name and where to save the file.

ui/browsing/albumpage.go

@@ -200,6 +200,9 @@ func NewAlbumPageHeader(page *AlbumPage) *AlbumPageHeader {
 				fyne.NewMenuItem("Add to playlist...", func() {
 					a.page.contr.DoAddTracksToPlaylistWorkflow(
 						sharedutil.TracksToIDs(a.page.tracks))
+				}),
+				fyne.NewMenuItem("Download", func() {
+					a.page.contr.ShowDownloadDialog(a.page.tracks)
 				}))
 			pop = widget.NewPopUpMenu(menu, fyne.CurrentApp().Driver().CanvasForObject(a))
 		}

ui/browsing/playlistpage.go

@@ -260,6 +260,9 @@ func NewPlaylistPageHeader(page *PlaylistPage) *PlaylistPageHeader {
 				fyne.NewMenuItem("Add to playlist...", func() {
 					a.page.contr.DoAddTracksToPlaylistWorkflow(
 						sharedutil.TracksToIDs(a.page.tracks))
+				}),
+				fyne.NewMenuItem("Download", func() {
+					a.page.contr.ShowDownloadDialog(a.page.tracks)
 				}))
 			pop = widget.NewPopUpMenu(menu, fyne.CurrentApp().Driver().CanvasForObject(a))
 		}